In order to approximate the appearance of continuous tone (photographic) images via ink-on-paper printing, the commercial printing industry relies on the process known as halftone printing. In halftone printing, color density gradations are produced by printing patterns of dots or areas of varying sizes, but of the same color density, instead of varying the color density continuously as is done in photographic printing.
There is an important commercial need to obtain a color proof image before a printing press run is made. It is desired that the color proof will accurately represent at least the details and color tone scale of the prints obtained from the printing press. In many cases, it is also desirable that the color proof accurately represent the image quality and halftone pattern of the prints obtained on the printing press. In the sequence of operations necessary to produce an ink-printed, full color picture, a proof is also required to check the accuracy of the color separation data from which the final three or more printing plates or cylinders are made. Traditionally, such color separations proofs have involved silver halide light-sensitive systems which require many exposure and processing steps before a final, full color picture is assembled.
In U.S. Pat. No. 5,126,760, a process is described for producing a direct digital, halftone color proof of an original image on a colorant-receiving element. The proof can then be used to represent a printed color image obtained from a printing press. The process described therein comprises:                a) generating a set of electrical signals which is representative of the shape and color scale of an original image;        b) contacting a colorant-donor element comprising a support having thereon a colorant layer and an infrared-absorbing material with a first colorant-receiving element comprising a support having thereon a polymeric, colorant image-receiving layer;        c) using the signals to image-wise heat by means of a diode laser the colorant-donor element, thereby transferring a colorant image to the first colorant-receiving element; and        d) retransferring the colorant image to a second colorant image-receiving element which has the same substrate as the printed color image. In the above process, multiple colorant-donors are used to obtain a complete range of colors in the proof. For example, for a full color proof, four colors—cyan, magenta, yellow and black are normally used.        
By using the above process, the image colorant is transferred by heating the colorant-donor containing the infrared-absorbing material with the diode laser to volatilize the colorant, the diode laser beam being modulated by the set of signals which is representative of the shape and color of the original image, so that the colorant is heated to cause volatilization only in those areas in which its presence is required on the colorant-receiving layer to reconstruct the original image.
In some instances, laser exposure of the donor element can result in the transfer of not only the desired colorant, but also that of the infrared absorbing material. If the infrared absorbing material has some visible absorption, this unwanted transfer can result in degraded color quality of the digital halftone image. It is common knowledge in the art to minimize the visible absorption of the infrared absorbing material through structural modification, thereby moderating the color degradation due to unwanted transfer of the infrared absorbing material. Another approach which is taught in EP 675003, U.S. Pat. No. 5,843,617, and related art is the use of a chemical agent capable of bleaching the infrared absorbing material to a visibly colorless state so it no longer degrades the desired colorimetry of the transferred image.
It has been found that the use of an infrared absorbing material which has a secondary, visible absorbance in the same spectral region as the visible colorant image transferred during laser thermal imaging, provides images displaying an improved color match and better saturation of color than images generated from donor elements in which the infrared absorber contributes to unwanted absorption. Thus, an infrared absorber with a lambda max>800 nm, and a secondary absorption peaking between 400 and 500 nm can provide images with excellent color fidelity of the newly generated image from a yellow donor. Likewise, the same infrared absorber as described in the previous sentence can be used in a magenta donor because the magenta image, and magenta printing inks in general, have significant absorption between 400-500 nm. These will be called color-matched examples of the invention, whereas examples in which the secondary visible absorption lies in a region outside the range of absorption of the transferred colorant, leading to degradation of image color, will be referred to as color-mismatched.
Similarly, using a thermal head in place of a diode laser as described in U.S. Pat. No. 4,923,846 can generate a thermal transfer proof. Commonly available thermal heads are not capable of generating halftone images of adequate resolution, but can produce high quality continuous tone proof images, which are satisfactory in many instances. U.S. Pat. No. 4,923,846 also discloses the choice of mixtures of colorants for use in thermal imaging proofing systems. Inkjet is also used as a low cost proofing method as described in U.S. Pat. No. 6,022,440. Likewise, an inkjet proof can be generated using combinations of either dispersed colorants in an aqueous fluid, or dissolved colorants in a solvent based system. U.S. Pat. No. 6,352,330 discloses methods for accomplishing this. Ink jet printers can also produce high quality continuous tone proof images, which by virtue of their cost are satisfactory in many instances. The colorants are selected on the basis of values for hue error and turbidity. The Graphic Arts Technical Foundation Research Report No. 38, “Color Material” (58-(5) 293-301, 1985) gives an account of this method.
An alternative and more precise method for color measurement and analysis uses the concept of uniform color space known as CIELAB, in which a sample is analyzed mathematically in terms of its spectrophotometric curve, the nature of the illuminant under which it is viewed, and the color vision of a standard observer. For a discussion of CIELAB and color measurement, see Principles of Color Technology, 2nd Edition, F. W. Billmeyer, pp.25-110, Wiley Interscience and Optical Radiation Measurements, Volume 2, F. Grum, pp. 33-145, Academic Press.
In using CIELAB, colors can be expressed in terms of three parameters: L*, a*, and b*, where L* is a lightness function, and a* and b* define a point in color space. Thus, a plot of a* versus b* values for a color sample can be used to accurately show where that sample lies in color space, i.e., what its hue is. This allows different samples to be compared for hue if they have similar density and L* values.
The colorants described in this invention have the following general formula:(IR)m-L-(VIS)nwherein:                L represents the non-chromophoric portions of the molecule and does not conjugate the first and second chromophores;        each IR chromophore independently represents a chromophore with λ-max above 700 nm;        each VIS chromophore independently represents a visible chromophore with λ-max from 400-700; and        m and n are independently 1-6.        
The visible chromophore can exhibit a λ-max in the range of 400-500 nm, in the range of 500-600 nm, or in the range of 600-700 nm.
The visible chromophore can be selected from yellow, cyan, and magenta. According to various embodiments, at least one visible chromophore can contain a pyrazoloazo group, a phenylazo group, a pyrazolodione group, an azoledione group, a pyrazolone group, a phenylamino group, a pyridinone group, an anilino group, a propene group, a cyclohexamine group, or a thiazoleazo group.
In color proofing in the printing industry, it is important to be able to match the proofing ink references provided by the International Prepress Proofing Association. In the United States, these ink references are density patches made with standard 4-color process inks and are known as SWOP® (Specifications Web Offset Publications) color aims. In 1995, ANSI CGATS TR 001-1995 was published which is becoming the standard in the United States industry. For additional information on color measurement of inks for web offset proofing, see “Advances in Printing Science and Technology”, Proceedings of the 19th International Conference of Printing Research Institutes, Eisenstadt, Austria, June 1987, J. T Ling and R. Warner, p.55.
It is desirable to provide proofs which can be used in parts of the world which do not use the SWOP® aims. For example, in Japan, a different standard is used and it would be desirable to provide a closer match to Japan Color. The 2001 Japan Color/Color Sample colorimetry values currently under consideration by the Japan National Committee for ISO/TC130 were used as the color reference.
It is a problem to be solved to provide a laser induced colorant donor element that employs an IR colorant that exhibits improved color matching.